The invention relates to alternating voltage detector.
Alternating voltage detectors, which are the starting point of the invention, are known in numerous applications in the state of the art. Such an alternating voltage detector is as a rule provided for detecting voltages in the range of about 50-600 VAC and 50-500 Hz. The alternating voltage detector in general consists of a sensor for detection, preferably without contact, of an electrical alternating field, an electrical signal transducer connected to the sensor for signaling an electrical alternating field detected by the sensor, and an indicator unit connected to the electrical signal transducer for indicating the signaled electrical alternating field. The indication of a detected alternating voltage takes place by the emission of an optical and/or an acoustic signal.
Such an alternating voltage detector is not only provided for voltage detection on plug sockets and the like; conductor breaks on exposed cables or cable drums are also to be traceable with this device. A problem with known commercial alternating voltage detectors is that these also detect electrostatic charges on housings, persons and other surfaces. If the detector detects a discharge (e.g., a spark) based on an electrostatic charge, this is indicated to the user by a short optical or acoustic signal. The impression can thereby arise that given portions are under voltage which are in fact free from voltage.
The invention thus has as its object to configure and develop the known alternating voltage detectors so that such an erroneous indication is no longer possible.
This object is attained according to the invention by an alternating voltage detector with the features of the characterizing portion of the appended independent claim.
Advantageous embodiments and developments of the invention are given in the dependent claims.
The essential concept of the invention consists in that an interference pulse filter is arranged between the sensor and the electrical signal transducer, and filters out electrical pulses or pulse packets detected by the sensor which have an average pulse duration or average pulse packet duration of less than a predetermined time. Thus the invention makes use of the fact that electrostatic charges on housings, persons and other surfaces discharge in a very short time. An erroneous interpretation of electrostatic charges is thereby substantially excluded.
Preferably, the interference pulse filter is designed so that electrical pulses are filtered out which have a pulse duration smaller than half the period of the electrical alternating field to be detected. This is of particular advantage when the discharge of the electrostatic charge takes place very quickly in comparison with an alternating pulse of an alternating voltage to be detected having a frequency between 50 and 500 Hz.
The sensor is preferably embodied as an antenna-like electrode which is insulated with respect to the surroundings. Such an arrangement permits a contactless alternating voltage detection.
In a particularly advantageous embodiment of the invention, a threshold circuit is connected following the sensor. This threshold circuit ensures that an alternating voltage signal is only indicated when a predetermined threshold voltage is exceeded. An indication thus takes place only in a predetermined frequency and voltage range of the alternating signal to be detected.
In a particularly advantageous embodiment of the invention, it is provided that the electrical signal transducer is a monostable flipflop. The reset time constant of the monostable is preferably to be chosen greater than the period of the alternating signal to be detected. This has the consequence that resetting of the monostable does not occur during the detection of such an alternating signal, but resetting of the monostable occurs in the case of the detection of an individual interference pulse. The output signal of the monostable is particularly suitable for driving an optical or acoustic signal transducer. In particular, it is provided that this monostable enables a pulse generator which produces an alternating signal which in its turn is used to produce an optical blinking or flickering signal or an acoustic signal.
In a particularly advantageous embodiment of the invention, it is provided that the series connection of the interference pulse filter and the monostable is realized in that between the input terminal and the output terminal, there is situated a first ohmic resistor which is in parallel with the series combination of a second ohmic resistor and a diode polarized in the forward direction, the resistance value of the first ohmic resistor being greater than that of the second ohmic resistor, and in that the output terminal is connected via a capacitor to a reference potential, the charging time constant given by the resistance value of the second ohmic resistor and the capacity of the capacitance being greater than half the period of the electric alternating field to be detected.